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A fume hood is a piece of laboratory equipment that is used to protect scientists from the possible negative effects of an experiment. This is primarily done by keeping the air inside the fume hood away from the people doing the experiment. Fume hoods also offer other features and protections that are useful in a laboratory setting.
Fume hoods have distinct physical characteristics. The base of the fume hood usually looks like a typical lab bench, and may have storage built into it, above the floor. On top of and connected to the bench is a very large enclosure called a cabinet. The cabinet will have a movable door, or sash, on the front that can be raised or lowered to provide access to the worktop portion of the fume hood. Openings around the edges of the worktop and/or at the top of the cabinet allow for the removal of gases in the cabinet. Experiments are performed inside the cabinet with the sash as low as possible so as to protect the experimenters.
Fume hoods come in two main types: recirculating and ducted. A recirculating fume hood draws the air from around an experiment into a built-in filter, then releases the purified air back into the lab or cabinet. A fan built into the cabinet is usually the primary draw in a recirculating fume hood. A ducted fume hood has the air drawn from it and into a ventilation system, where it is cleaned and then released outside of the building. There are usually multiple ducted fume hoods attached to such a vent system.
Fume hoods provide protection from more than just dangerous gases. The enclosed nature of the cabinet helps to protect experimenters from being hurt by debris if something explodes or splashes. Fume hoods are also usually made of flame-retardant materials, so unexpected fires tend not to be as damaging as they would be in an open area.
Most fume hoods have numerous features built into them to assist with experimentation. Vapor-sealed lights allow experimenters to see what they are doing. Electrical outlets both inside and outside the fume hood are common, and allow for electronic devices to be easily used during the experiment. Vacuum pumps are also common, and allow for low-pressure or vacuum experiments to be conducted. Gas outlets that can be connected to various tanks and flows are useful for specific experiments, as are water faucets and drains.
An alternative to a fume hood is a device called a fume arm. Also known as a snorkel vacuum, these long, tube-like devices can be positioned over experiments in an open lab environment in order to draw away any dangerous gases. Multiple fume arms are usually found in a lab equipped with them, and they are almost always hooked up to a central ventilation system or air purifier. Fume arms are much more mobile and compact than fume hoods, and also cost significantly less. They have many uses, but are most often found in academic labs where many experiments are being performed at once on long lab benches.